Rarely has a dead-end vote caused this much trouble for U.S. Sen. Mark Udall.
But on Wednesday the Colorado Democrat was forced, once again, to pick sides in the fight over the Keystone pipeline — an issue that has stalked his re-election campaign for months.
A self-proclaimed moderate,...

Rarely has a dead-end vote caused this much trouble for U.S. Sen. Mark Udall.

But on Wednesday the Colorado Democrat was forced, once again, to pick sides in the fight over the Keystone pipeline — an issue that has stalked his re-election campaign for months.

+ captionU.S. Sen. Mark Udall, D-Colo., waves to delegates after accepting the nomination to run for his seat in the November 2014 election during the Colorado Democratic Party's State Assembly in Denver on Saturday, April 12, 2014. (AP Photo/David Zalubowski) ORG XMIT: CODZ101

A self-proclaimed moderate, Udall has tried to court both environmentalists and the energy industry as he attempts to win a second term against U.S. Rep. Cory Gardner, R-Yuma.

But this week that strategy hit a bump: the ambitions of U.S. Sen. Mary Landrieu of Louisiana, another vulnerable Democrat up for re-election.